The present invention relates generally to fabrics for use on papermaking machines and more particularly to Fourdrinier, or forming fabrics.
In the manufacture of sheetlike structures of materials such as paper, kraft, board, pulp, asbestos and the like, it is common practice to utilize a Fourdrinier papermaking machine having an endless belt of a generally flat fabric formed from woven metal wires or synthetic polymeric yarns of materials such as polyamides, polyesters and the like. The synthetic yarns can be either monofilament or multifilament yarns.
In operation of the machine, an aqueous suspension of pulp fibers is deposited onto the moving forming fabric. As the forming fabric travels, some of the water content of the suspension drains through the porous fabric so as to form a generally self-supporting continuous web of material. The drainage of the water through the belt may be assisted by mechanisms such as suction boxes or the like. As the web reaches the end of the belt, the web is "picked up" from the forming fabric and is subsequently subjected to a series of rolling and drying operations to yield the final sheet-like product.
The woven forming fabric of the papermaking machine must possess a variety of characteristics to properly function in the papermaking process. The forming fabric should be sufficiently fluid permeable so as to allow for the drainage of water from the fiber suspension but sufficiently closed so as to prevent the passage of significant amounts of fibers contained in the suspension. Since the forming fabric is under tension and is subjected to bending as it passes over the rolls, the strength and resistance to fatigue and wear of the fabric must be high so as to insure the longest possible working life. In addition, the strands of fabric should be dimensionally stable relative to one another so that the fabric will not have varying drainage characteristics which can affect the uniformity of the final product.
Various types of weaving patterns for forming fabrics for use in papermaking machines are known in the art. Generally these patterns have the identical symmetry in the warp and shute directions. For example, U.S. Pat. No. 3,858,623 to Lefkowitz discloses a prior art pattern wherein the warp and weft, or shute, yarns pass over one yarn, beneath the two adjacent yarns and then over the next yarn. For convenience, this pattern may be referred to as a one-two type pattern. The above-mentioned patent further discloses another pattern for papermaking machine fabrics which can be defined as a one-three type pattern. In addition, U.S. Pat. No. 3,211,606 to Watson discloses a papermaking fabric of woven wires having a repetitive four strand, two-two type pattern and U.S. Pat. No. 1,927,498 to Lindsay discloses a one-two type pattern for a woven metal wire belt for a papermaking machine. Similar types of papermaking fabrics are also disclosed in U.S. Pat. Nos. 2,903,201 to Halden et al, 2,755,047 to Henke, 3,139,119 to Buchanan, 3,143,150 to Buchanan, 3,159,530 to Heller et al and 3,421,230 to Ward.
Various problems are associated with the use of the above-mentioned fabrics as belts in papermaking machines. For example, the fabric having the so-called one-three type of repetitive pattern in the weave tends to have one relatively smooth surface and one relatively rough surface. Such fabric tends to have relatively short life when run with the smooth surface against the suction boxes. For example, the one-three type fabrics that are used in the production of heavier papers typically only have a usable life of about thirty days at normal production speeds. The smoother surface of the fabric also tends to erode the covers of the suction box in a papermaking machine which therefore requires frequent grinding of the covers to maintain adequate suction of the belt. Furthermore, such fabrics do not allow the dewatered web to be "picked up" or released easily from the smoother surface when they are run with the rougher surface against the suction box covers. The same problems are encountered to a greater or lesser extent in the use of the other fabric weave types such as a two-two or a one-two. These other fabric weave types also may have inadequate drainage characteristics.